1. Field of the Invention
The invention relates to a fluid actuated device for converting linear-to-rotary motion capable of delivering a constant torque output without a side thrust force vector being imposed upon on the driving piston.
2. Description of the Prior Art
A number of known thrust generating devices for converting linear motion to rotary motion are commercially available. However, each of the known devices has certain inherent problems.
For example, the conventional crank arm system has an inherent torque output capacity which is not constant. The torque output is at its lowest point at the beginning and end of each stroke of a crank arm system. This is undesirable in the utilization of this system in the actuation of rotary valves, for example, where the valve resistance is greatest at the beginning and the end of each cycle during the opening and closing thereof. By varying the length of the connecting rod in proportion to the radius of the valve motor, the torque curve may be changed. Additionally, in the event the connecting rod length is shortened to improve the torque curve of the system, the side thrust imposed on the actuating piston is increased. The increased side thrust on the actuating piston thus increases the wear on the piston and shortens the operating life of the system.
Another system heretofore utilized is the conventional rack and pinion system wherein only a few teeth of the rack and pinion gears are engaged at any given time and therefore are highly stressed under conditions requiring high torque output. If long life and efficient operation are required for such a system, the rack and the associated pinion must be aligned with extreme precision, at all times. Under high torque requirements, the rack and pinion system, including the operator therefor, must be enlarged. Accordingly, alignment becomes increasingly important. Manifestly, under high loading conditions, distortions may cause significant misalignment of the cooperating elements. Precision alignment of the associated piston, cylinder, and gears is necessary, and if not maintained, may result in undue seal, piston, cylinder, and gear wear. In order to maintain a constant torque output curve, a rack and pinion type actuating system must employ thrust bearings in order to assure proper alignment.
The scotch yoke mechanism is another well known torque generating device for converting linear motion into rotary motion. Typically, this system converts linear movement of a fluid pressure actuated piston and piston rod by a cam follower being integrally affixed to the piston rod and adapted to slide in a slot in a rotatable lever arm. The torque output curve normally is nonlinear and closely matches the torque requirements of typical valves. Due to the inherent and substantial side thrust characteristics of the scotch yoke mechanism, fluid pressure actuators must incorporate costly thrust bearings and large piston-cylinder clearances to prevent excessive piston wear. Clearly, thrust bearings effect the overall efficiency of the system thereby reducing torque available and necessary for operation of the actuator and manipulation of the valve.
Another torque generating device is a vane mechanism utilizing an impeller. In order for the impeller to function properly, the integrity of the seal between the impeller and the associated cylinder wall must be carefully maintained. Because of the complex design and sealing problems of the vane mechanism, the device may require frequent servicing.
The present invention provides a device for converting linear to rotary motion with reliable opening and closing forces for plug or ball type valves. This device is compact in design and provides a constant high torque output characteristic by utilizing a fluid pressure actuated motor having a piston which does not experience side loading forces and thus minimizes the wear of the piston seal assembly and the piston and cylinder areas. Furthermore, the apparatus of the present invention is completely sealed to protect the operating elements from external contamination and operating personnel from injury from moving parts.